extends doesn't disrupt such connections, nor even the ability to observe them. This is in part about the gap between code that static analysis can classify as unused vs. code that current static analysis does classify as unused, and also about interplay between authors, tools, and the language itself. As a demonstration, consider expanding the example from above:

export class C {
  /* constructor and "core" members */
  #counter = 0n;
  constructor(initial = 0n) { this.#counter = BigInt(initial); }
  adjustBy(n) { this.#counter += BigInt(n); }
  get counter() { return this.#counter; }
};
const { prototype: CShakable } = class extends C {
  /* tree-shakable methods */
  increment() { this.adjustBy(1n); return this.counter; }
  decrement() { this.adjustBy(-1n); return this.counter; }
};
export const { increment, decrement } = CShakable;

import { C, increment } from "c.js";
console.log(increment.call(new C(99n))); // 100n

export const { prototype: { increment, decrement } } = class ...

or

class Methods extends C { ... }
export const { increment, decrement } = Methods.prototype;

My first outreach attempt (Three.js): https://discourse.threejs.org/t/im-on-tc39-would-three-js-be-interested-in-tree-shakable-methods-syntax/90519

My takeaways:

1. Don't reply more than once per week
2. Ignore replies that consist of off-topic ratholes and feelings
3. Tweak OP to address common questions/misunderstandings

Feel free to judge my attempt mercilessly so I can do better next time. Thx :)